The present invention relates to embossed multi-lamina cellulosic fibrous structures, particularly the process of producing embossed multi-lamina cellulosic fibrous structures having selective autogeneous bond sites.
Cellulosic fibrous structures are a staple of everyday life. Cellulosic fibrous structures such as tissue paper are used as consumer products for paper towels, toilet tissue, facial tissue, napkins and the like. The large demand for such paper products has created a demand for improved versions of the products and the methods of their manufacture.
Manufacturers have concentrated improvements in softness, bulkiness, absorbency and aesthetics of cellulosic fibrous structures. For softness, attention has been primarily focused on chemical softening agents. For bulkiness, absorbency, and aesthetics, manufacturers have centered on multi-lamina substrates, particularly embossed multi-lamina substrates.
There have been numerous attempts to reduce the abrasive effects of tissue products through the addition of chemical softening agents (also referred to as xe2x80x9cchemical softenersxe2x80x9d). As used herein, the term xe2x80x9cchemical softening agentxe2x80x9d refers to any chemical ingredient which improves the tactile sensation perceived by the consumer who holds a particular paper product and rubs it across the skin. Although somewhat desirable for towel products, softness is a particularly important property for facial and toilet tissues. Such tactilely perceivable softness can be characterized by, but is not limited to, friction, flexibility, and smoothness, as well as subjective descriptors, such as a feeling like velvet, silk or flannel. Suitable materials include those which impart a lubricious feel to tissue. This includes, for exemplary purposes only, basic waxes such as paraffin and beeswax and oils such as mineral oil and silicone oil as well as petrolatum and more complex lubricants and emollients such as quaternary ammonium compounds with long alkyl chains, tertiary amines, functional silicones, fatty acids, fatty alcohols and fatty esters.
Multi-lamina laminate substrates are well known in the art of consumer products. Such products are typically cellulosic fibrous structures having more than one, typically two, laminae superimposed in face-to-face relationship to form a laminate. It is known in the art to emboss the laminate for aesthetic purposes and to produce bonds between the laminae. Embossing can also increase the surface area of the laminae thereby enhancing their bulk and water holding capacity.
Embossing is typically performed by one of two processes, knob-to-knob embossing or nested embossing. Knob-to-knob embossing consists of axially parallel rolls juxtaposed to form a nip between the crests of the embossing knobs on opposing rolls. Nested embossing consists of axially parallel rolls juxtaposed to form a nip where the embossing knobs on one roll mesh between the embossing knobs of the other roll. Examples of knob-to-knob embossing and nested embossing are illustrated in the prior art by U.S. Pat. No. 3,414,459 issued Dec. 3, 1968 to Wells and commonly assigned; U.S. Pat. No. 3,547,723 issued Dec. 15, 1970 to Gresham; U.S. Pat. No. 3,556,907 issued Jan. 19, 1971 to Nystrand; U.S. Pat. No. 3,708,366 issued Jan. 2, 1973 to Donnelly; U.S. Pat. No. 3,738,905 issued Jun. 12, 1973 to Thomas; U.S. Pat. No. 3,867,225 issued Feb. 18, 1975 to Nystrand and U.S. Pat. No. 4,483,728 issued Nov. 20, 1984 to Bauernfeind.
During the embossing process, the laminae are fed through separate nips formed between separate embossing rolls and pressure rolls where embossing knobs on the embossing rolls produce compressed regions in the laminae. The two laminae are then fed through a common nip formed between the embossing rolls where the embossing knobs on the two rolls bring the laminae together in a face-to-face contacting relationship.
Nested embossing has proven to be the preferred process for producing embossed multi-lamina laminates. Products produced by nested embossing exhibit a softer more quilted appearance that is maintained throughout the balance of the converting process, and packaging. With nested embossing, the crests of the embossing knobs on one embossing roll intermesh with the embossing knobs on the opposing embossing roll at the nip formed between the two rolls. This causes the patterns produced on the two laminae transported therebetween to intermesh enabling the embossed sites produced on one lamina to provide support for the embossed sites produced on the other lamina.
With nested embossing an adhesive applicator roll is typically aligned axially parallel with one of the two embossing rolls forming a nip therewith upstream of the nip formed between the two embossing rolls. The adhesive applicator roll transfers adhesive to the lamina on the embossing roll at the crests of the embossing knobs. The crests of the embossing knobs typically do not touch the perimeter of the opposing roll at the nip formed therebetween necessitating the addition of a marrying roll to apply pressure for lamination. The marrying roll forms a nip with the same embossing roll forming the nip with the adhesive applicator roll, downstream of the nip formed between the two embossing rolls. Typical marrying rolls have a smooth continuous surface resulting in the lamination of every potential laminating point as shown in U.S. Pat. No 3,867,225 issued Feb. 18, 1975 to Nystrand.
A preferred means for embossing and bonding multiple laminae of tissue in a face-to-face relationship involves embossing autogeneously (without adhesives) by high pressure lamination. With high pressure lamination, the adhesive applicator roll is eliminated and the marrying roll is replaced with a steel anvil roll. In addition to bonding the laminae, high pressure lamination produces a visually distinctive embossment pattern exhibiting a glassine appearance which is decoratively pleasing. High pressure lamination is disclosed in U.S. Pat. No. 3,377,224 issued Apr. 9, 1968 to Gresham et al and U.S. Pat. No. 3,323,983 issued Sep. 8, 1964 to Palmer. Both patents are incorporated herein by reference.
The High pressure lamination typically requires pressures ranging from about 40,000 psi to about 80,000 psi to produce adequate bond strength between the laminae. For laminae treated with chemical softening agents, the required pressures can exceed 150,000 psi.
High laminating pressures can induce fatigue on the surface of an embossing roll limiting the useful life of the roll. What""s more, high laminating pressures can damage the substrate by tearing or puncturing the laminae. Thus, there is a desire to minimize the lamination pressure required for bonding laminae by high pressure lamination, particularly, laminae treated with chemical softening agents.
U.S. Pat. No. 4,481,243 issued Nov. 6, 1984 to Allen, incorporated herein by reference, discloses a tissue comprising a planar substrate carrying an emollient where the substrate comprises at least two laminae united by embossments without adhesives. Allen addressed the issue of bonding, without adhesives, laminae treated with emollient by limiting the embossed sites to regions free of the emollient.
The present invention provides a means for bonding laminae by high pressure lamination at reduced pressures by adding a functional fluid such as water to selective bond sites prior to bonding. The process is capable of autogeneously bonding laminae uniformly treated with chemical softeners at reasonable pressures without causing damage to the laminae or reducing the useful life of the embossing roll.
The invention provides a process for producing a multi-lamina cellulosic substrate bonded at discrete bond sites by high pressure lamination at reduced pressures. A fluid applicator roll operates in conjunction with a pattern roll to increase the local moisture level at selective bond sites prior to high pressure lamination. The selective bond sites may be continuous or discrete. The process is applicable to multi-lamina substrates having laminae previously treated with chemical softening agents.
In one embodiment, a fluid applicator roll is juxtaposed axially parallel to a pattern roll forming a fluid transfer nip therewith. The pattern roll is also juxtaposed axially parallel to a steel anvil roll forming a bonding nip therewith. A first lamina is transported relative to the fluid transfer nip where it is selectively wetted at continuous or discrete locations by the fluid applicator roll. The first lamina is then transported to the bonding nip where it is bonded in a face-to-face relationship with a second lamina by high pressure lamination.
In an alternate embodiment, the process comprises providing a first lamina embosser and a second lamina embosser. The first lamina embosser comprises a first pressure roll juxtaposed axially parallel to a first embossing roll forming a first embossing nip therewith and the second lamina embosser comprises a second pressure roll juxtaposed axially parallel to a second embossing roll forming a second embossing nip therewith. The first and second embossing rolls comprise a plurality of radially oriented embossing knobs projecting from a periphery to form crests. The first lamina embosser and the second lamina embosser are positioned in a parallel arrangement such that the first embossing roll forms an intermeshing nip with the second embossing roll. A fluid applicator roll is provided juxtaposed axially parallel to the first embossing roll forming a fluid transfer nip therewith upstream of the intermeshing nip. An anvil roll is provided juxtaposed axially parallel with the first embossing roll forming a bonding nip therewith downstream of the intermeshing nip.
As the first and second laminae are transported relative to the first and second lamina embossers, the fluid applicator roll applies water to the first lamina at selective locations corresponding to embossed sites produced at the first embossing nip. The first and second laminae are transported to the intermeshing nip and assembled in a nested a face-to-face relationship forming a multi-lamina substrate. The multi-lamina substrate is then transported to the bonding nip where the two laminae are bonded at the wetted embossed sites by high pressure lamination.